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Debt - Preferred Securities (Details) - USD ($) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
NYM Preferred Trust I |  |
Debt Instrument [Line Items] |  |
Principal value of trust preferred securities | $ 25,000,000 | $ 25,000,000
Interest rate | Three month LIBOR plus 3.75%, resetting quarterly | Three month LIBOR plus 3.75%, resetting quarterly
Scheduled maturity | March 30, 2035 | March 30, 2035
NYM Preferred Trust I | LIBOR |  |
Debt Instrument [Line Items] |  |
Interest rate, basis spread | 3.75% | 3.75%
NYM Preferred Trust II |  |
Debt Instrument [Line Items] |  |
Principal value of trust preferred securities | $ 20,000,000 | $ 20,000,000
Interest rate | Three month LIBOR plus 3.95%, resetting quarterly | Three month LIBOR plus 3.95%, resetting quarterly
Scheduled maturity | October 30, 2035 | October 30, 2035
NYM Preferred Trust II | LIBOR |  |
Debt Instrument [Line Items] |  |
Interest rate, basis spread | 3.95% | 3.95%

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